1. Field of the Invention
This invention is directed to highly sensitive or ultrasensitive equipment and methods that detect trace amounts of gases by refined optical illumination and response detection as well as photoacoustic methods.
2. Description of the Related Art
Mid and long wave infrared quantum cascade lasers (QCLs) cover a very important spectral region from about 3 μm to 15 μm where most of the important trace gas pollutants, chemical warfare agents, toxic industrial chemicals, and vapors of explosives exhibit their characteristic infrared fingerprint absorption. Use of these QCLs for sensitive spectral analysis of the target gases requires a broad tunability of essentially single wavelength radiation and techniques for detection of the target gases at very low concentrations in the presence of normally occurring interferent gases whose infrared absorption fingerprints overlap with those of the targets.
Quantum cascade lasers as fabricated operate as Fabry-Perot cavity lasers formed by the end facets of the semiconductor laser chips and produce a multiwavelength output covering some hundreds of nanometers. The spectral position of each of the independently lasing wavelengths is determined by the Fabry-Perot cavity modes of the laser chip and the wavelength spread is determined by the gain width of the laser. Such broad spectral output is virtually useless for the highly sensitive and selective detection of the target gases. The laser output needs to be essentially one single wavelength and mechanisms are needed for broad tunability so that the fingerprint characteristic of the target gas absorption can be accurately measured. The broad tuning necessary for the sensitive and selective target gas detection has led to both software and hardware innovations.
Further, use of broadly tunable single wavelength radiation for sensitive detection of the target gases in a sample is complicated by the presence of other constituents, often called interferents, in the sample. The overlapping spectra signatures of the interferents and the target gases, obtained using broadly tunable single frequency lasers can be deconvolved using algorithms and techniques described in an earlier patent application, U.S. patent application Ser. No. 11/256,377 filed Oct. 21, 2005 for System and Method for High Sensitivity Optical Detection of Gases which shares co-inventors with this instant patent document and which is incorporated herein by this reference thereto.